Dual-Camera Spherical Indentation System for Examining the Mechanical Characteristics of Hydrogels

Abstract

The mechanical and viscoelastic properties of the hydrogels are important in determining their suitability for biological applications. An advanced three-dimensional imaging technique has now been developed to examine the mechanical properties of these hydrogels under spherical indentation. A dual-camera spherical indentation system has been set-up and alginate hydrogels 20mm in diameter and between 400µm and 600µm in thickness were examined. A small number of PVC particles (75 –100µm in diameter) were suspended throughout the hydrogels. Each hydrogel was suspended around it outer edge while submerged in water. A stainless steel sphere was placed centrally on the hydrogel surface causing it to deform. A CCD camera connected to a long working distance microscope recorded the deformation displacement. A second camera positioned above the sample holder recorded the resulting displacement of particles. The images from both cameras were analyzed and used to establish the mechanical behavior of the hydrogel. This dual-camera approach allowed the distribution of localized strain at different points in the hydrogel to be examined during deformation. The mechanical properties of the hydrogels can be calculated from the deformation data using inverse finite element modeling based on a large deformation theory. This technique has several advantages over conventional mechanical characterization techniques including nondestructive measurements, hydrogels can be maintained under sterile or cell culture conditions during measurement, repeated measurement can be taken of the same sample at different time points and there is no backing substrate effect during indentation. This approach has many potential applications including determination of the elastic or viscoelastic properties of hydrogels, examination of the strain distribution during deformation and detection of local defects or abnormalities in the materials mechanical properties.